The upper limb is probably the part of the body that is most involved with the external environment. The one thing that differentiates us from most animals is the opposable thumb, which has evolved over many years to be a handy tool in the daily function of all humans. We use the upper limb for important functions such as feeding and protection from dangers. As the upper limb consists of muscles, they are controlled by nerves. Thus, it is important for us to be aware of the nerve routes and innervation of the upper limb.
The nervous system is divided into the Central nervous system and peripheral nervoous system. In the peripheral nervous system, there is somatic and autonomic. Autonomic sympathetic fibers coming out from gray rami of C5 to T1 level supply the sweat glands and the errectore pilli muscles of the skin in the upper limb. As for the somatic, anterior rami of nerve fibers coming from the C5 to T1 levels form a plexus called the brachial plexus. the brachial plexux has majour contribution to the innervation of the upper limb.
The brachial plexus runs starts from in between the anterior scalene and the middle scalene muscles in the neck. These are known as the roots of the brachial plexus. the long thoracic nerve (C5, C6, C7) branches out from the upper three roots which innervates the serratus anterior. Branches to the longus colli and scalene muscles branches out from each of the five roots. Dorsal scapular nerve (C5) innervates the rhomboids, and levator scapulae muscle.
The five roots form 3 trunks. C5 and C6 combines to form the superior trunk. C7 continues as the middle trunk. C8 and T1 forms the inferior trunk. The trunk lies on the first rib.
The superior trunk has two nerve branching out; suprascapular nerve (C5 and C6) and the nerve to the sublavius (C5, C6).
The three trunks each form an anterior and posterior division. All the posterior divisions combine to form the posterior cord. The anterior devision of the superior and middle trunks form the lateral cord. The anterior divisions of the inferior trunk continue as the medial cord.
The cords are names in accordance to its relations with the axillary artery. The lateral cord is lateral to the artery, medial cord medial to it and the posterior cord posterior to it.
The branches of the lateral cord are the lateral pectoral nerve (C5, C6, C7) that innervates the sternal part of the pectoralis major muscle and the musculocutaneous nerve (C5, C6, C7) that innervates the flexor compartment of the arm namely the biceps brachii, coracobrachialis and the brachialis.
The posterior cord gives rise to the radial nerve ( C5, C6, C7, C8, T1). The radial nerve innervates the posterior compartment of the forearm and arm. They include the triceps, anconeus, part of brachialis, extensor carpi radialis longus via deep radial nerve branch. the muscles of the forearm includes the supinator, extensor carpi radialis brevis, extensor carpi ulnaris, extensor digitorum, extensor digiti minimi, extensor indices, abductor policis longus, extensor policis longus and extensor policis brevis.
The posteror cord also gives rise to the axillary nerve (C5, C6) that innervates the deltoids and teres minor muscle.
Other nerves from the posterior cord are the upper subscapular nerve (C5, C6) and the thoracodorsal nerve (C6, C7, C8) that innervates the subscapularis muscle and latissimus dorsi muscle respectively.
The medial nerve gives rise to the medial pectoral nerve (C8, T1), the medial cutaneous nerve of arm (C8, t1, T2), medial cutaneous nerve of forearm (C8, T1) and ulanr nerve ( C8, T1) that sdupplies the hypothenar eminence, the flexor carpi ulnaris and the medial half of flexor carpi profundas.
A division from the medial and lateral cord forms the median nerve that innervates all the flexor muscles of the forearm namely the flexor carpi radialis, flexor carpi superficialis, pronator teres, lateral half of flexor carpi profundas and the thenar eminence of the hand.